Ground fault circuit interruption (GFCI) devices have been sanctioned by the National Electric Code for use in residential circuits to protect against the hazards of electric shock. Such GFCI devices, as presently commercially available, utilize a differential current transformer to sense a current imbalance in the line and neutral conductors occasioned by line fault current returning to the source through an unintended ground circuit path other than the neutral conductor. To prevent injurious electrical shock, the differential current transformer must develop a signal voltage of sufficient magnitude to enable a signal processor to initiate circuit interruption when the current differential in the line and neutral conductors is five milliamps or more. For ease of manufacture and to provide a compact design, the line and neutral conductors, which constitute primary windings of the differential current transformer, each make a single pass through the aperture of the toroidal transformer core. Thus, to satisfy a 5 milliamp trip level, the signal processor must be designed to respond to a transformer primary excitation of 0.005 ampere-turns. So that the design constraints on the signal processor are not so rigid as to be prohibitively expensive, the differential transformers must have a high permeability core and a secondary winding of many turns -- typically in excess of 1000 turns of very fine wire -- in order to develop signal voltages of practical magnitudes. Signal levels are nevertheless quite low, 1 to 10 millivolts, requiring high amplification. With such high amplification, the processor design must insure amplifier stability and adequate noise immunity to prevent nuisance tripping of the GFCI device.
It is accordingly an object of the present invention to provide a sensor for developing a large signal voltage in response to a small current imbalance in a pair of conductors.
Another object of the invention is to provide an improved ground fault circuit interruption (GFCI) device incorporating a differential current sensor of the above character.
Yet another object is to provide a GFCI device of the above character which is less expensive to manufacture and more reliable in operation.
Other objects of the invention will be made apparent from the following detailed description and claims.